1. Field of the Invention
The present invention relates generally to nuclear reactors and, more particularly, is concerned with a reconstitutable control rod spider assembly having removable control rods with detachable split upper end plugs.
2. Description of the Prior Art
In a typical nuclear reactor, such as a pressurized water type, the reactor core includes a multiplicity of fuel assemblies. Each fuel assembly is composed of top and bottom nozzles with a plurality of elongated transversely spaced guide thimbles extending longitudinally between and attached at opposite ends to the nozzles. Also, a plurality of transverse support grids are axially spaced along and attached to the guide thimbles. Further, a plurality of elongated fuel elements or rods transversely spaced apart from one another and from the guide thimbles are supported by the transverse grids between the top and bottom nozzles. The fuel rods each contain fissile material and are grouped together in an array which is organized so as to provide a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Since the rate of heat generation in the reactor core is proportional to the nuclear fission rate, and this, in turn, is determined by the neutron flux in the core, control of heat generation at reactor start-up, during its operation and at shutdown is achieved by varying the neutron flux. Generally, this is done by absorbing excess neutrons using control rods which contain neutron absorbing material. The guide thimbles, in addition to being structural elements of the fuel assembly, also provide channels for insertion of the neutron absorber control rods within the reactor core. The level of neutron flux and thus the heat output of the core is normally regulated by the movement of the control rods into and from the guide thimbles.
One common arrangement utilizing control rods in association with a fuel assembly can be seen in U.S. Pat. No. 4,326,919 to Hill. This patent shows a control rod spider assembly which includes a plurality of control rods and a spider structure supporting the control rods at their upper ends. The spider structure, in turn, is connected to a control drive mechanism that vertically raises and lowers (referred to as a stepping action) the control rods into and out of the hollow guide thimbles of the fuel assembly. The typical construction of the control rod used in such an arrangement is in the form of an elongated metallic cladding tube having a neutron absorbing material disposed within the tube and with end plugs at opposite ends thereof for sealing the absorber material within the tube.
The spider structure typically includes a plurality of radially extending vanes supported on and circumferentially spaced about a central hub. The vanes are flat metal plates positioned on edge and being connected at their inner ends to the central hub. Cylindrical shaped control rod connecting fingers are mounted to and supported by the vanes, with some of the vanes having only a single connecting finger and other vanes having a spaced pair of connecting fingers associated therewith.
Typically, the upper end plug of each control rod has a threaded outer end which is receivable into a bore in the lower portion of one finger of the spider structure and threadable into a tapped hole formed in the finger at the inner end of the bore. The end plug is then secured or locked therein by a key or pin inserted into the side of the finger and the end plug and then welded therein. Generally speaking, the above-described control rod spider assembly is not reconstitutable, that is, the assembly cannot readily be taken apart and worn or damaged components replaced. Instead, the whole assembly must be discarded.
In many older nuclear reactor plants, the control rods are approaching the end of their design life. Furthermore, in a number of newer plants with modified reactor internals, the control rods are experiencing accelerated wear. In most cases wear occurs over local areas on the cladding of some, but not all, control rods of a given control rod spider assembly. Wear is believed to be caused by coolant flow-induced vibration. The severity of the clad wear is dependent upon the type of nuclear plant and internals design and typically occurs only on a limited number of control rods of a given assembly.
Consequently, there is a growing interest in and need for reconstitutable control rod spider assemblies wherein selected individual control rods that are prematurely worn or damaged can be removed and replaced. A spider assembly which allows removal of a control rod from a spider structure by severing or cutting off the threadably fastened top end of the control rod and the fastener is disclosed in French patent application No. 86/08381. A spider assembly which seems to allow removal of a control rod from a spider by removing a set screw and unthreading the upper end plug of the control rod from a support rod axially movably mounted to a spider vane finger is disclosed in Nakazato U.S. Pat. No. 4,711,756. Spider assemblies which allow removal of a control rod from a spider structure by deforming a locking element on the top end of the control rod are disclosed in U.S. patents to Edwards et al U.S. Pat. No. 4,314,885 and Walton U.S. Pat. No. 4,381,283. However, none of these disclosures would appear to be the optimum approach to providing a reconstitutable control rod spider assembly.